Anodic oxide films have been employed commercially for over 100 years. These films find use in a variety of industrial applications, including electrolytic capacitors, rectifiers for converting alternating current to direct current, lightning arrestors, insulation on aluminum and aluminum alloy motor and transformer windings, as decorative coatings on furniture and appliances, as decorative coatings on niobium and titanium jewelry, and as a hard wear surface on aluminum or titanium machine and aircraft parts.
Anodic oxide films have traditionally been categorized as belonging to one of two basic types of film. The first type is the non-barrier or decorative type of film. These oxide films are usually grown on aluminum, titanium, or alloys thereof in electrolyte solutions which partially dissolve the oxide film.
Anodic aluminum films grown in cold sulfate or phosphoric acid solutions are porous, having a very large number of pores, generally of hexagonal shape, through which the electrolyte is in contact with the base metal (through a relatively thin oxide layer at the bottom of each pore) and supplies oxygen for continued anodic oxide growth so long as current is supplied. These films are usually grown with less than 50 volts applied across the anodizing cell. The pores in these films readily accept a wide variety of dyes, and they may be exposed to dye during or after the anodizing process. The pores for both decorative and wear-resistant anodic films on aluminum or its alloys are usually sealed by exposure to solutions which cause the pores to fill with a bulky aluminum oxide hydration product. Nickel acetate solutions have frequently been used to seal decorative and wear surfaces on aluminum.
Decorative anodic films on titanium are usually produced in cold sulfuric acid electrolyte solutions. Although these films are less porous than decorative films on aluminum and tend to be more uniform in thickness, they tend to be of a lamellar structure and are sometimes present as a series of very thin layers connected at many points and appearing uniform and continuous to the naked eye. The uniformity of thickness and transparency of anodic films on titanium produced in cold sulfuric acid solutions results in a vivid series of interference colors, similar to those characteristic of the so-called barrier anodic films on tantalum, so that no dyes are required to produce decorative results. The lamellar structure of these films, mentioned above, probably accounts for the observation that they tend to not be as effective as thermally produced films for the purposes of wear or corrosion resistance.
The second basic type of anodic oxide film is the barrier film. This type of anodic oxide is generally produced in electrolyte solutions which are relatively non-corrosive toward the substrate metals upon which the films are grown although barrier films may be produced on aluminum in electrolyte solutions which have significant solvent action on the hydrated forms of the oxide, such as borate solutions. Barrier anodic oxide films tend to be very uniform in thickness with the thickness being directly proportional to the applied voltage and the absolute (Kelvin) temperature of the electrolyte solution as described by Torissi (Relation of Color to Certain Characteristics of Anodic Tantalum Films, Journal of the Electrochemical Society, Vol. 102, No. 4, April 1955, pp. 176-180).
Barrier anodic oxide films age down to very low current values when held at constant voltage in barrier film forming electrolytes, in contrast to non-barrier films which grow thicker as long as voltage is applied. Barrier anodic oxide films also exhibit the property of rectification; they are highly insulating with the base metal positive relative to the electrolyte solution and readily pass electric current with the base metal biased negative relative to the electrolyte solution. The rectification or electronic valve action has led to the name valve metals, for the group of metals upon which anodic films can be grown which exhibit this property. Barrier anodic oxide films have traditionally been limited to relatively thin layers, generally well under a micron in thickness. This is due to the extremely small amount of barrier oxide produced per volt applied, 10-25 angstroms per volt depending upon the valve metal. This results in electric fields of up to 10,000,000 volts/cm across the thickness of the oxide. In order to prevent electron avalanche failure of barrier anodic oxide films at these high field levels, it has been found necessary to employ higher resistivity electrolytes to produce higher voltage films. The breakdown voltage of these films has been found to be proportional to the logarithm of the electrolyte resistivity. Electron avalanche failure of barrier films generally limits the maximum voltage to well under 1,000 volts or less than one micron in thickness. The maximum voltage obtained with traditional barrier film anodizing techniques is approximately 1,500 volts, obtained by Lilienfeld (U.S. Pat. Nos. 1,986,779 and 2,013,564) using polyglycol borate electrolytes, which produced barrier oxide films on aluminum of approximately 1.5 microns in thickness.
It has been recognized for some time that, for some applications in the electronics, aerospace, and chemical industries, it would be very useful to have the capability of producing very thick barrier-type anodic oxide films. It has also been widely recognized that a method of producing very thick (i.e., over one micron thick) barrier oxide films capable of withstanding very high applied voltages (i.e., over 500 volts) with relatively low anodizing voltage is highly desirable. Just such an anodizing method was developed in 1997 and is the subject of U.S. Pat. Nos. 5,837,121 and 5,935,408, Kinard et. al., as well as co-pending application Ser. No. 09/090,164, now U.S. Pat. No. 6,149,793 and Ser. No. 09/265,593.
This method of producing barrier-type anodic oxide films of unlimited thickness on valve metals at relatively low anodizing cell voltages (dubbed, Non-Thickness-Limited or N-T-L anodizing by the inventors) was also described in a technical paper, The Non-Thickness-Limited Growth of Anodic Oxide Films on Valve Metals, published in Electrochemical and Solid State Letters, Vol. 1, No. 3, September 1998, pp. 126-129.
Non-Thickness-Limited anodizing, as described in U.S. Pat. Nos. 5,837,121 and 5,935,408, Kinard et. al., consists of the application of relatively low voltage (about 30 volts or less) to a valve metal object immersed in a glycerine solution of dibasic potassium phosphate containing less than about 0.1% water and at a temperature above about 150.degree. C. in order to produce a barrier anodic oxide film on the surface of the valve metal object. Basic salts, other than dibasic potassium phosphate, were found to result in fairly rapid polymerization of the glycerine to polyglycerine accompanied by the evolution of water.
It was found that thermally stable acid salts giving a solution pH of 4-7 may be employed (in place of the dibasic potassium phosphate) in combination with the glycerine solvent for non-thickness-limited anodizing of valve metals, as described in co-pending application Ser. No. 09/090,164.
It was found that, after a period of days at temperatures above 150.degree. C., the glycerine-based electrolyte solutions employed for non-thickness-limited anodizing contain so little water (below 0.05%) that the N-T-L anodizing may prove difficult to initiate. It was found that a thin anodic oxide film applied to the valve metal substrate prior to N-T-L anodizing, such as a 3-volt anodic oxide film applied in room temperature dilute phosphoric acid, provides a film sufficiently thick to then be converted readily to non-thickness-limited anodizing kinetics upon immersion in an N-T-L electrolyte above 150.degree. C. and applying voltage (i.e., the valve metal substrate with the preformed film gives rise to N-T-L anodizing more readily in low water content N-T-L electrolytes than does a valve metal substrate without a thin pre-formed film). This phenomena is described in co-pending application Ser. No. 09/265,593, which is primarily concerned with the use of constant current anodizing to produce a predictable anodic oxide film thickness under non-thickness-limited anodizing conditions.
Unfortunately, some valve metals, most notably niobium and niobium alloys, have proven difficult to anodize under non-thickness-limited anodizing kinetics due to the difficulty in initiating N-T-L film growth with these materials. The electronic leakage current through the native or passive oxide film which forms on the surface of niobium and alloys such as Nb/1%Zr is sufficiently high that little or no ionic current (necessary for anodic film growth) flows through the passive film upon application of voltage in N-T-L anodizing solutions at the required temperatures (i.e., in 10 wt. % dibasic potassium phosphate solution in glycerine containing less than 0.1 wt. % water and maintained at a temperature above 150.degree. C.).